Ok so I have a question fro you pros on here:

I have been getting into the habit of pre-thinning my Tamiya jars when I get them. I learned the trick from Andys Hobby Headquarters™ on YouTube. Has been super handy but I have seem to forgotten which ones I have thinned and which ones I haven't. How do you tell if paint has been thinned by looking at it?

I know about the "milk consistency" but the paint is already like that before its thinned. Is there any sure-fire way of telling without putting it in the AB and spraying it?

It sounds like it is not thoroughly mixed, but why not try it full strength in your ab?

I commented on the "Andys Hobby HQ" method a year or so ago--it's not really a good method for thinning because the paint can never be used again for anything but airbrushing at that specific mixture-- basically if you want another formulation later, you're going to need to buy another jar of paint. That being said, I do mix my paint sometimes in large quantities (for me, anything the size of a jar is a large quantity). In those cases I use a wide tip Sharpee and write "PTA" right on the side and the top of the lid-- for "Pre-Thinned - Airbrush". I'm not sure what kind of modeler Andy is, but I am not impressed with some of his videos-- the Wingnut Wings Albatros rigged with stretch elastic from a fabric store was really a sacrilege for a WnW kit. And his mixing the Tamiya paints is another example of wasting paint--too expensive for me. If you must know, the formula was to add thinner to the shoulder of a brand new bottle-- not very scientific or accurate, and it doesn't allow for evaporation, age, and quantities of paint already formulated. In estimation, it looks like a 10:3 (10 part paint to 3 part thinner)-- unsuitable for many airbrush applications ( I usually thin Tamiya at 6:4 for basic coverage, and as high as 3:7 or greater for fine line painting).
VR, Russ

Stick a piece of sprue in the bottle and pull it up. You'll be able to tell the difference between thinned and unthinned by how thick it sticks to the sprue. Then label the bottle with a Sharpie!

Russ is spot on this one. Without knowing a mix ratio for your paints, you’ll never get good performance from most airbrushes. Paint colors from any given manufacturer are almost never all the same viscosity, and most paints in a line have to be thinned to a specific ratio based on the nozzle size you spray with and what pressure you use.

Additionally, if need to mix colors, which is very common when using Tamiya paints, your pre thinned paints won’t yield the correct hues most of the time because they don’t have the correct pigment density.

Obviously, you can proceed as you wish and your results may be superb, but I would personally ditch the suspect paint, and replace it with new. I would also recommend mixing and thinning the paints as needed in a separate pill cup or bottle, and never mix thinned paint back into your base paint. Your paints will last much longer since you won’t be wasting as much, spray better and mix to the correct hue when needed.

Cheers
Paul H

All good info in this thread!!

The main reason I was mixing in the jars was the mess. I was mixing small batches to use but then I would run out of paint half way through a job, have to remix more, and continue spraying. Sometimes I would mix too much and have to toss a half cup of paint.

While not as precise as some people like, it has worked pretty ok for me. I am marking the bottles as I mix them, so I don't mix all of them.

I appreciate all the info from you guys! That's why I ask the pros!

Charlie.
Another, and perhaps more important reason not to mix paint in the original jar is that you will invariably reduce the shelf life of the paint by diluting the pigment to carrier mix as Paul stated. This is especially true if you use non-proprietary thinners. Paint is formulated at specific carrier to pigment ratios (all paint, from house paint to artist oil paints) to give the user the ability to obtain the appropriate color density and coverage, based on the chosen application method. The end result of mixing it in the original jar will inevitably be more wasted paint. My suggestion is to mix paint in batches, using an appropriate mixing cup and pippette or eye dropper. You can save empty paint jars and then transfer your mixture into them if you want to save it for later use, or If you need more, just mix more. This keeps your original jar of paint viable for a longer period of time. There is one exception, and that is Gunze Mr. Color lacquers-- this paint is highly pigmented (concentrated) and is made to be heavily diluted with Gunze Mr. Leveler thinner before use-- which makes the concentrated pigment/carrier mix go farther. It's the only paint I mix in the jar, unless I'm making a large batch of tamiya to use in a short period of time--then I'll use the transfer method. I have a large stock of old Tamiya jars on hand just for that purpose--I clean them out with lacquer thinner and remove the labels, using the Sharpee to write on the lids. But otherwise, letting paint sit in an original jar with thinner will eventually alter the chemical formulation of the paint, making it unsuitable for other uses, or will ruin the paint completely. Andy is either unaware of this, or doesn't care since part of his business is to sell paint.
VR, Russ

Just reading the whole conversation. And I must say, it really interests me. Since I'm looking for other paint options for replacing my current paints or Revell over time.